In this time of reduced funding for basic science combined with vast progress in data manipulation, many researchers use data from previous scientific studies, or big databases collected over time on items such as weather, pollution, diseases, voting patterns, and crime, to elicit new predictions and hypotheses.  I imagine that’s how this conclusion was derived:

Scientists at University College London released a study concluding that the Little Ice Age, a period of global cooling that began in the late 1500s, was a direct consequence of the colonization of the Americas, in which approximately 10 percent of the world’s population died and an area the size of France was reforested. “This is useful; it shows us what reforestation can do,” said coauthor Chris Brierley. “That kind of reduction is worth perhaps just two years of fossil fuel emissions at the present rate.” 

During my docent-led tours at the Seymour Center I have occasion to discuss the UC Natural Reserve system, comprised of 39 reserves totaling over 700,000 acres. Each university manages some of the reserves, with UCSC responsible for four. Most are not open to the public, though a few allow occasional guided access.

The reserves were set aside to ensure undisturbed ecosystems would be available for training and research in a time of steady ecosystem destruction. These sites are where the next generation of ecologists is trained to use spotting scopes and take samples in the wild. Experiments on ecosystems may extend through seasons or even years, and setting them up in the reserves ensures the results aren’t skewed by inadvertent tampering.

The excerpt above describes an experiment beyond the wildest dreams of any UC researcher, both for its scale and because it would be completely unethical. Imagine the summary line of the grant application:  Infect two continents with disease vectors with a high mortality rate, then slowly repopulate them, allowing time for forests to regrow, while monitoring the global climate.

Today the six billion of us experiment on a grand scale, destroying rain forests, eating processed food, and getting most of our information through social media, and no one is even slightly surprised to think that there are enough of us to create global natural and cultural changes. It’s a little more impressive (terrifying?) that only 500 million of us caused an Ice Age, even a Little one.

A lot of viruses  helped.